Jan 4 Suncor Energy Inc's top executive said on Monday it was "conceivable but highly improbable" that Suncor will extend a C$4.3 billion ($3.08 billion) hostile takeover bid for Canadian Oil Sands Ltd beyond this week's deadline.

Canada's biggest oil producer launched a bid for Canadian Oil Sands in October and last month extended the bid until Friday after the Alberta Securities Commission gave shareholders until Jan. 4 to decide how to respond.

Canadian Oil Sands is the largest-interest owner in Alberta's Syncrude project, the country's largest single-source producer oil producer.

Suncor reiterated its all-stock offer of 0.25 of Suncor share for each Canadian Oil Sands share on Monday, implying a value of C$8.93 per share based on Suncor's Dec. 31 close.

Chief Executive Steve Williams said in an interview that the process had been going on too long and Suncor would walk away if not enough shares were tendered by the end of the week.

"We can only invest so much time and money in this effort and will feel compelled to move on to other opportunities if we don't see substantial support for our bid on Friday," he said.

Suncor is looking at other asset purchase opportunities in regions where it already has operations such as the oil sands, offshore eastern Canada and the North Sea, Williams added.

He said there would be no more sweeteners to the bid given the weak macroeconomic environment. Oil prices have dropped around 70 percent since June 2014.

"The price is full and fair and were we to launch the bid today it would not be at this level," Williams said.

In a letter to shareholders on Monday, Canadian Oil Sands Chairman Don Lowry once again urged shareholders reject the offer.

"Suncor's substantially undervalued bid is set to lapse, and when it does they say they will walk away. For all of us, as shareholders, this scenario reveals a far more compelling and valuable alternative: Independence," Lowry said.

The letter comes as Canadian Oil Sands' shareholder rights plan expires. The company adopted the plan, also known as poison pill, two days after Suncor made the offer in early October.

Lowry said Canadian Oil Sands had also considered a full range of alternatives against the Suncor offer, including a full or partial sale to other parties and a royalty financing, and had the financial resources to weather the current oil price downturn.

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